1. Field of the Invention
The present invention relates an apparatus for spot welding superposed workpieces by putting the superposed workpieces between a pair of electrodes and applying a welding current between the electrodes under application of pressure.
2. Description of the Related Art
In a spot welding apparatus, the superposed workpieces are put between edges of a pair of electrodes and pressed between the electrodes under application of a welding current (electric current) whereby the superposed workpieces are spot welded. In case material of the workpieces is apt to be deformed by the application of pressure between the electrodes 55, 56, two workpieces 51, 52 can be contacted each other by deforming them as shown in FIG. 3(b) even if there is opening 53 between the two work-pieces 51, 52 owing to variability of the dimension precision of their materials. Therefore the two workpieces 51, 52 can be firmly spot welded with forming a nugget 58 at the welding portions.
In a spot welding apparatus used in production line of vehicles, a high-tension steel having high strength is frequently used as materials of vehicles with increase of requirement making them lightweight and high strength in recent years. However, the application of pressure by the electrodes does not bring about sufficient deformation of workpieces made of the high-tension steel, and therefore it becomes difficult that the two work-pieces are firmly spot welded when an opening is produced between the two workpieces of the high-tension steel.
JA2002-961178 discloses a spot welding apparatus in which spot welding is carried out with taking the opening between the two workpieces into consideration. In the spot welding apparatus, the two workpieces are put between a pair of electrodes under application of pressure, and welding current is applied between the electrodes under the application of pressure for a short time to preheat the workpieces so as to be intimate with each other, and then the workpieces are spot welded each other with controlling the welding current such that calorific value of the workpieces reaches a predetermined value within a predetermined period time for applying the current.